Rod Austin, a newspaper photolithographer and an amateur
astronomer who has discovered three comets, all of which are
named after him, responded.

As comets move slowly against the background stars, it takes many
weeks for sufficient observations to be obtained to determine exactly
what a comet will do. Comet Shoemaker-Levy 9 was actually discovered
on March 24 1993, probably more than twenty years after it had been
captured by Jupiter and eight months after its previous close approach
to Jupiter during which the large gravitational forces had broken it up
into many pieces, some about a kilometre across and each similar to a
'dirty snowball'.

Over a few days in July of 1994 the very large pieces of the original
nucleus, now travelling at a speed in excess of 200,000 kilometres per
hour, buried themselves deep into the atmosphere of Jupiter before the
extreme temperatures and pressures caused them to explode like gigantic
depth-charges. Each explosion caused huge volumes of gas from deep in
the atmosphere to burst to the surface after several hours, causing
huge dark markings to appear in the upper atmosphere of Jupiter. These
were most impressive in even quite small telescopes. Some of the areas
were larger than the Earth, and the volume of atmosphere affected probably
exceeded the volume of all the water in the Pacific Ocean.

It is a timely reminder of what has happened on Earth in the past, and
will eventually happen again.